R.F. Wassenaar

CMOS low-voltage operational amplifiers with constant-G/sub m/ rail-to-rail input stage

Two 3.3-V operational amplifiers with constant-gm rail-to-rail input stage and rail-to-rail output stage are presented. The constant transconductance (gm) ensures a constant unity-gain frequency within the whole commonmode input range. Two new methods to control thegm are introduced. Both operational amplifiers use the same rail-to-rail output stage. The operational amplifiers have been integrated in a CMOS semicustom process with transistor lengths of 10µm. The common-mode input voltage swing extends beyond the positive supply rail by 400 mV and beyond the negative supply rail by 200 mV. The output voltage is able to reach within 130 mV of the supply rails. The output current of the operational amplifiers is 2 mA and the voltage gain is 85 dB. The unity-gain frquency is 165 kHz, which is mainly limited by the relatively long transistor lengths of 10µm. In another process with channel lengths of 2µm, simulation results showed that a unity-gain frequency of 4 MHz can easily be obtained.

A low-voltage CMOS op amp with a rail-to-rail constant-g/sub m/ input stage and a class AB rail-to-rail output stage

A low-voltage two-stage operational amplifier (op-amp) is presented. The op-amp features rail-to-rail operation and has an input stage with a constant transconductance (g/sub m/) over the entire common-mode input range. The input stage consists of an n- and a p-MOS differential pair connected in parallel. The constant g/sub m/ is accomplished by regulating the tail-currents with the aid of an MOS translinear (MTL) circuit. The resulting g/sub m/ is constant within 5%. The common-source output stage employs a feedback circuit which also contains an MTL circuit. This feedback circuit ensures class AB operation and prevents the transistors in the output stage from cutting off. The op-amp isi realized in a semi-custom CMOS process with minimum channel lengths of 10/spl mu/m. Simulations show that the minimum supply voltage is less than 2.5 V. A unity gain bandwidth of 550 kHz and a DC voltage gain larger than 80 dB are feasible. The input range exceeds the supply rails, whereas the output range reaches the rails within 130 mV.<<ETX>>

New techniques for high-frequency RMS-to-DC conversion based on a multifunctional V-to-I convertor

Two bipolar RMS-DC convertor circuits of the computing type which require no rectifier function are discussed. Improved frequency response is thus obtained. RMS-to-DC computation is carried out in the current domain. To make the circuit suitable for voltage driving, a dedicated V-to-I convertor is developed. Measured 1% bandwidths of the RMS-to-DC convertors are 35 and 22 MHz, respectively. Conversion error is less than 1% for the crest factors up to five. >